<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    .left{
      /* width: 100px; */
      height: 100px;
      background-color: pink;
      float: left;
    }
    .right{
      /* 开启bfc */
      overflow: hidden;
      height: 300px;
      background-color: blue;
    }
  </style>
</head>
<body>
  <div class="left">123</div>
  <div class="right">123</div>
</body>
</html>